1. Field of the Invention
The present invention relates to an improvement in magnetic recording medium for video tape recording.
2. Description of the Prior Art
Ferromagnetic thin film type magnetic recording medium manufactured by forming such a ferromagnetic thin film as of Fe, Co, Ni or their alloys on a polyester film, by means of vacuum deposition, ion plating, spattering, or the like method is known. These recording media are advantageous in that their recording density can be drastically increased in comparison with the conventional coated-magnetic-film type recording media. For such high density recording, the pole gap of the magnetic recording head must be made very small and the surface of the magnetic recording medium must be very smooth in order to decrease spacing loss to the utmost extent. However, when the tape surface is too smooth, it has been found that squealing of the tape is induced when running around a recording head drum, and therefore the surface state must be finished with good control. Since the thickness of the ferromagnetic thin film is only 100 .ANG.-5000 .ANG., the surface condition of the magnetic recording medium is greatly influenced by the surface configuration of the substrate plastic film. Accordingly there have been many proposals to control the surface conditions of the substrate film surfaces. One of the proposals is disclosed in the Japanese Patent unexamined publication No. Sho 53-107314 disclosing that a magnetic recording medium made by forming a ferromagnetic thin film on a substrate polyester film having thermo-plastic minute granular protrusions has satisfactory tape running characteristics and also a good reproduced picture quality. But this conventional magnetic recording medium has a problem that when surface roughness of the ferromagnetic recording medium is made under 300 .ANG. in order to improve the surface smoothness of the magnetic recording medium or attaining a high recording density, stick slips are produced thereby inducing the squealing.
In order to eliminate the squealing of the tape running around the head drum, many proposals have been made, for example, in the Japanese Patent unexamined publications No. Sho 52-18770, No. Sho 52-84264, No. Sho 54-40883, No. Sho 53-116115, No. Sho 53-128685, No. Sho 54-94574, No. Sho 56-10455, No. Sho 56-16937. In these documents, it is proposed that the characteristic of touching the head and running is intended to be improved by evenly forming a relatively finely roughened surface, for instance, wrinkle shape protrusions, minute earth worm shape protrusions or minute granular shape protrusions, to improve running and head cylinder touching characteristics at the same time. The above-mentioned improvements are very effective in improving the characteristics of head touching, squealing and running in normal temperature and normal moisture conditions.
However, when the magnetic recording medium is used in adverse conditions of high moisture and high temperature, that is, 30.degree. C. and 80-90% RH, even the above-mentioned magnetic recording medium of the type proposed is liable to make a squealing noise at the rotary head cylinder part and also the still lives thereof are not sufficiently long. In order to solve the problem, some of the inventors of the present invention proposed in Japanese Patent Application No. Sho 57-128784, filed on July 22, 1982, utilizing polyester film as a substrate and making polyester oligomers included in the polyester substrate film to separate and form minute protrusions on the substrate surface by a thermal treatment at the time or before forming the ferromagnetic metal thin film on the substrate. But in this proposed method, it is an important problem to evenly distribute the oligomer protrusions on the surface by some appropriate measures.